BESS Workgroup A. Sajassi (Editor)
INTERNET-DRAFT Cisco
Intended Status: Standards Track J. Drake (Editor)
Juniper
N. Bitar
Nokia
R. Shekhar
Juniper
J. Uttaro
AT&T
W. Henderickx
Nokia
Expires: September 27, 2017 March 27, 2017
A Network Virtualization Overlay Solution using EVPNdraft-ietf-bess-evpn-overlay-08
Abstract
This document describes how Ethernet VPN (EVPN) can be used as an
Network Virtualization Overlay (NVO) solution and explores the
various tunnel encapsulation options over IP and their impact on the
EVPN control-plane and procedures. In particular, the following
encapsulation options are analyzed: VXLAN, NVGRE, and MPLS over GRE.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
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INTERNET DRAFT EVPN Overlay March 27, 20171 Introduction
In the context of this document, a Network Virtualization Overlay
(NVO) is a solution to address the requirements of a multi-tenant
data center, especially one with virtualized hosts, e.g., Virtual
Machines (VMs) or virtual workloads. The key requirements of such a
solution, as described in [Problem-Statement], are:
- Isolation of network traffic per tenant
- Support for a large number of tenants (tens or hundreds of
thousands)
- Extending L2 connectivity among different VMs belonging to a given
tenant segment (subnet) across different PODs within a data center or
between different data centers
- Allowing a given VM to move between different physical points of
attachment within a given L2 segment
The underlay network for NVO solutions is assumed to provide IP
connectivity between NVO endpoints (NVEs).
This document describes how Ethernet VPN (EVPN) can be used as an NVO
solution and explores applicability of EVPN functions and procedures.
In particular, it describes the various tunnel encapsulation options
for EVPN over IP, and their impact on the EVPN control-plane and
procedures for two main scenarios:
a) single-homing NVEs - when a NVE resides in the hypervisor, and
b) multi-homing NVEs - when a NVE resides in a Top of Rack (ToR)
device
The possible encapsulation options for EVPN overlays that are
analyzed in this document are:
- VXLAN and NVGRE
- MPLS over GRE
Before getting into the description of the different encapsulation
options for EVPN over IP, it is important to highlight the EVPN
solution's main features, how those features are currently supported,
and any impact that the encapsulation has on those features.
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INTERNET DRAFT EVPN Overlay March 27, 20172 Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [KEYWORDS].
3 Terminology
Most of the terminology used in this documents comes from [RFC7432]
and [NVO3-FRWK].
NVO: Network Virtualization Overlay
NVE: Network Virtualization Endpoint
VNI: Virtual Network Identifier (for VXLAN)
VSID: Virtual Subnet Identifier (for NVGRE)
EVPN: Ethernet VPN
EVI: An EVPN instance spanning the Provider Edge (PE) devices
participating in that EVPN.
MAC-VRF: A Virtual Routing and Forwarding table for Media Access
Control (MAC) addresses on a PE.
Ethernet Segment (ES): When a customer site (device or network) is
connected to one or more PEs via a set of Ethernet links, then that
set of links is referred to as an 'Ethernet segment'.
Ethernet Segment Identifier (ESI): A unique non-zero identifier that
identifies an Ethernet segment is called an 'Ethernet Segment
Identifier'.
Ethernet Tag: An Ethernet tag identifies a particular broadcast
domain, e.g., a VLAN. An EVPN instance consists of one or more
broadcast domains.
PE: Provider Edge device.
Single-Active Redundancy Mode: When only a single PE, among all the
PEs attached to an Ethernet segment, is allowed to forward traffic
to/from that Ethernet segment for a given VLAN, then the Ethernet
segment is defined to be operating in Single-Active redundancy mode.
All-Active Redundancy Mode: When all PEs attached to an Ethernet
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segment are allowed to forward known unicast traffic to/from that
Ethernet segment for a given VLAN, then the Ethernet segment is
defined to be operating in All-Active redundancy mode.
4 EVPN Features
EVPN was originally designed to support the requirements detailed in
[RFC7209] and therefore has the following attributes which directly
address control plane scaling and ease of deployment issues.
1) Control plane information is distributed with BGP and Broadcast
and Multicast traffic is sent using a shared multicast tree or with
ingress replication.
2) Control plane learning is used for MAC (and IP) addresses instead
of data plane learning. The latter requires the flooding of unknown
unicast and ARP frames; whereas, the former does not require any
flooding.
3) Route Reflectors are used to reduce a full mesh of BGP sessions
among PE devices to a single BGP session between a PE and the RR.
Furthermore, RR hierarchy can be leveraged to scale the number of BGP
routes on the RR.
4) Auto-discovery via BGP is used to discover PE devices
participating in a given VPN, PE devices participating in a given
redundancy group, tunnel encapsulation types, multicast tunnel type,
multicast members, etc.
5) All-Active multihoming is used. This allows a given customer
device (CE) to have multiple links to multiple PEs, and traffic
to/from that CE fully utilizes all of these links.
6) When a link between a CE and a PE fails, the PEs for that EVI are
notified of the failure via the withdrawal of a single EVPN route.
This allows those PEs to remove the withdrawing PE as a next hop for
every MAC address associated with the failed link. This is termed
'mass withdrawal'.
7) BGP route filtering and constrained route distribution are
leveraged to ensure that the control plane traffic for a given EVI is
only distributed to the PEs in that EVI.
8) When a 802.1Q interface is used between a CE and a PE, each of the
VLAN ID (VID) on that interface can be mapped onto a bridge table
(for upto 4094 such bridge tables). All these bridge tables may be
mapped onto a single MAC-VRF (in case of VLAN-aware bundle service).
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9) VM Mobility mechanisms ensure that all PEs in a given EVI know
the ES with which a given VM, as identified by its MAC and IP
addresses, is currently associated.
10) Route Targets are used to allow the operator (or customer) to
define a spectrum of logical network topologies including mesh, hub &
spoke, and extranets (e.g., a VPN whose sites are owned by different
enterprises), without the need for proprietary software or the aid of
other virtual or physical devices.
Because the design goal for NVO is millions of instances per common
physical infrastructure, the scaling properties of the control plane
for NVO are extremely important. EVPN and the extensions described
herein, are designed with this level of scalability in mind.
5 Encapsulation Options for EVPN Overlays5.1 VXLAN/NVGRE Encapsulation
Both VXLAN and NVGRE are examples of technologies that provide a data
plane encapsulation which is used to transport a packet over the
common physical IP infrastructure between Network Virtualization
Edges (NVEs) - e.g., VXLAN Tunnel End Points (VTEPs) in VXLAN
network. Both of these technologies include the identifier of the
specific NVO instance, Virtual Network Identifier (VNI) in VXLAN and
Virtual Subnet Identifier (VSID) in NVGRE, in each packet. In the
remainder of this document we use VNI as the representation for NVO
instance with the understanding that VSID can equally be used if the
encapsulation is NVGRE unless it is stated otherwise.
Note that a Provider Edge (PE) is equivalent to a NVE/VTEP.
VXLAN encapsulation is based on UDP, with an 8-byte header following
the UDP header. VXLAN provides a 24-bit VNI, which typically provides
a one-to-one mapping to the tenant VLAN ID, as described in
[RFC7348]. In this scenario, the ingress VTEP does not include an
inner VLAN tag on the encapsulated frame, and the egress VTEP
discards the frames with an inner VLAN tag. This mode of operation in
[RFC7348] maps to VLAN Based Service in [RFC7432], where a tenant
VLAN ID gets mapped to an EVPN instance (EVI).
VXLAN also provides an option of including an inner VLAN tag in the
encapsulated frame, if explicitly configured at the VTEP. This mode
of operation can map to VLAN Bundle Service in [RFC7432] because all
the tenant's tagged frames map to a single bridge table / MAC-VRF,
and the inner VLAN tag is not used for lookup by the disposition PE
when performing VXLAN decapsulation as described in section 6 of
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[RFC7348].
[NVGRE] encapsulation is based on GRE encapsulation and it mandates
the inclusion of the optional GRE Key field which carries the VSID.
There is a one-to-one mapping between the VSID and the tenant VLAN
ID, as described in [NVGRE] and the inclusion of an inner VLAN tag is
prohibited. This mode of operation in [NVGRE] maps to VLAN Based
Service in [RFC7432].
As described in the next section there is no change to the encoding
of EVPN routes to support VXLAN or NVGRE encapsulation except for the
use of the BGP Encapsulation extended community to indicate the
encapsulation type (e.g., VxLAN or NVGRE). However, there is
potential impact to the EVPN procedures depending on where the NVE is
located (i.e., in hypervisor or TOR) and whether multi-homing
capabilities are required.
5.1.1 Virtual Identifiers Scope
Although VNIs are defined as 24-bit globally unique values, there are
scenarios in which it is desirable to use a locally significant value
for VNI, especially in the context of data center interconnect:
5.1.1.1 Data Center Interconnect with Gateway
In the case where NVEs in different data centers need to be
interconnected, and the NVEs need to use VNIs as a globally unique
identifiers within a data center, then a Gateway needs to be employed
at the edge of the data center network. This is because the Gateway
will provide the functionality of translating the VNI when crossing
network boundaries, which may align with operator span of control
boundaries. As an example, consider the network of Figure 1 below.
Assume there are three network operators: one for each of the DC1,
DC2 and WAN networks. The Gateways at the edge of the data centers
are responsible for translating the VNIs between the values used in
each of the data center networks and the values used in the WAN.
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1. Option 1: Single Broadcast Domain per EVI
In this option, a single Ethernet broadcast domain (e.g., subnet)
represented by a VNI is mapped to a unique EVI. This corresponds to
the VLAN Based service in [RFC7432], where a tenant-facing interface,
logical interface (e.g., represented by a VLAN ID) or physical, gets
mapped to an EVPN instance (EVI). As such, a BGP RD and RT are needed
per VNI on every NVE. The advantage of this model is that it allows
the BGP RT constraint mechanisms to be used in order to limit the
propagation and import of routes to only the NVEs that are interested
in a given VNI. The disadvantage of this model may be the
provisioning overhead if RD and RT are not derived automatically from
VNI.
In this option, the MAC-VRF table is identified by the RT in the
control plane and by the VNI in the data-plane. In this option, the
specific MAC-VRF table corresponds to only a single bridge table.
2. Option 2: Multiple Broadcast Domains per EVI
In this option, multiple subnets each represented by a unique VNI are
mapped to a single EVI. For example, if a tenant has multiple
segments/subnets each represented by a VNI, then all the VNIs for
that tenant are mapped to a single EVI - e.g., the EVI in this case
represents the tenant and not a subnet . This corresponds to the
VLAN-aware bundle service in [RFC7432]. The advantage of this model
is that it doesn't require the provisioning of RD/RT per VNI.
However, this is a moot point when compared to option 1 where auto-
derivation is used. The disadvantage of this model is that routes
would be imported by NVEs that may not be interested in a given VNI.
In this option the MAC-VRF table is identified by the RT in the
control plane and a specific bridge table for that MAC-VRF is
identified by the <RT, Ethernet Tag ID> in the control plane. In this
option, the VNI in the data-plane is sufficient to identify a
specific bridge table.
5.1.2.1 Auto Derivation of RT
When the option of a single VNI per EVI is used, in order to simplify
configuration, the RT used for EVPN can be auto-derived. RD can be
auto generated as described in [RFC7432] and RT can be auto-derived
as described next.
Since a gateway PE as depicted in figure-1 participates in both the
DCN and WAN BGP sessions, it is important that when RT values are
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auto-derived from VNIs, there is no conflict in RT spaces between DCN
and WAN networks assuming that both are operating within the same AS.
Also, there can be scenarios where both VXLAN and NVGRE
encapsulations may be needed within the same DCN and their
corresponding VNIs are administered independently which means VNI
spaces can overlap. In order to ensure that no such conflict in RT
spaces arises, RT values for DCNs are auto-derived as follow:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AS # |A| TYPE| D-ID | Service ID |
+-----------------------------------------------+---------------+
| Service ID (Cont.) |
+-------------------------------+
- 2 bytes of global admin field of the RT is set to the AS number.
- Three least significant bytes of the local admin field of the RT is
set to the VNI, VSID, I-SID, or VID.
- The most significant bit of the local admin field of the RT is set
as follow:
0: auto-derived
1: manually-derived
- The next 3 bits of the most significant byte of the local admin
field of the RT identifies the space in which the other 3 bytes are
defined. The following spaces are defined:
0 : VID (802.1Q VLAN ID)
1 : VXLAN
2 : NVGRE
3 : I-SID
4 : EVI
5 : dual-VID (QinQ VLAN ID)
- The remaining 4 bits of the most significant byte of the local
admin field of the RT identifies the domain-id. The default value of
domain-id is zero indicating that only a single numbering space exist
for a given technology. However, if there are more than one number
space exist for a given technology (e.g., overlapping VXLAN spaces),
then each of the number spaces need to be identify by their
corresponding domain-id starting from 1.
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In EVPN, an MPLS label for instance identifying forwarding table is
distributed by the egress PE via the EVPN control plane and is placed
in the MPLS header of a given packet by the ingress PE. This label is
used upon receipt of that packet by the egress PE for disposition of
that packet. This is very similar to the use of the VNI by the egress
NVE, with the difference being that an MPLS label has local
significance while a VNI typically has global significance.
Accordingly, and specifically to support the option of locally-
assigned VNIs, the MPLS Label1 field in the MAC/IP Advertisement
route, the MPLS label field in the Ethernet AD per EVI route, and the
MPLS label field in the PMSI Tunnel Attribute of the Inclusive
Multicast Ethernet Tag (IMET) route are used to carry the VNI. For
the balance of this memo, the above MPLS label fields will be
referred to as the VNI field. The VNI field is used for both local
and global VNIs, and for either case the entire 24-bit field is used
to encode the VNI value.
For the VLAN-based service (a single VNI per MAC-VRF), the Ethernet
Tag field in the MAC/IP Advertisement, Ethernet AD per EVI, and IMET
route MUST be set to zero just as in the VLAN Based service in
[RFC7432].
For the VLAN-aware bundle service (multiple VNIs per MAC-VRF with
each VNI associated with its own bridge table), the Ethernet Tag
field in the MAC Advertisement, Ethernet AD per EVI, and IMET route
MUST identify a bridge table within a MAC-VRF and the set of Ethernet
Tags for that EVI needs to be configured consistently on all PEs
within that EVI. For locally-assigned VNIs, the value advertised in
the Ethernet Tag field MUST be set to a VID just as in the VLAN-aware
bundle service in [RFC7432]. Such setting must be done consistently
on all PE devices participating in that EVI within a given domain.
For global VNIs, the value advertised in the Ethernet Tag field
SHOULD be set to a VNI as long as it matches the existing semantics
of the Ethernet Tag, i.e., it identifies a bridge table within a MAC-
VRF and the set of VNIs are configured consistently on each PE in
that EVI.
In order to indicate which type of data plane encapsulation (i.e.,
VXLAN, NVGRE, MPLS, or MPLS in GRE) is to be used, the BGP
Encapsulation extended community defined in [TUNNEL-ENCAP] and
[RFC5512] is included with all EVPN routes (i.e. MAC Advertisement,
Ethernet AD per EVI, Ethernet AD per ESI, Inclusive Multicast
Ethernet Tag, and Ethernet Segment) advertised by an egress PE. Five
new values have been assigned by IANA to extend the list of
encapsulation types defined in [TUNNEL-ENCAP] and they are listed in
section 13.
The MPLS encapsulation tunnel type, listed in section 13, is needed
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in order to distinguish between an advertising node that only
supports non-MPLS encapsulations and one that supports MPLS and non-
MPLS encapsulations. An advertising node that only supports MPLS
encapsulation does not need to advertise any encapsulation tunnel
types; i.e., if the BGP Encapsulation extended community is not
present, then either MPLS encapsulation or a statically configured
encapsulation is assumed.
The Ethernet Segment and Ethernet AD per ESI routes MAY be advertised
with multiple encapsulation types as long as they use the same EVPN
multi-homing procedures (section 8.3.1, Split Horizon) - e.g., the
mix of VXLAN and NVGRE encapsulation types is a valid one but not the
mix of VXLAN and MPLS encapsulation types.
The Next Hop field of the MP_REACH_NLRI attribute of the route MUST
be set to the IPv4 or IPv6 address of the NVE. The remaining fields
in each route are set as per [RFC7432].
Note that the procedure defined here to use the MPLS Label field to
carry the VNI in the presence of a Tunnel Encapsulation Extended
Community specifying the use of a VNI, is aligned with the procedures
described in section 8.2.2.2 of [tunnel-encap] ("When a Valid VNI has
not been Signaled").
5.2 MPLS over GRE
The EVPN data-plane is modeled as an EVPN MPLS client layer sitting
over an MPLS PSN-tunnel server layer. Some of the EVPN functions
(split-horizon, aliasing, and backup-path) are tied to the MPLS
client layer. If MPLS over GRE encapsulation is used, then the EVPN
MPLS client layer can be carried over an IP PSN tunnel transparently.
Therefore, there is no impact to the EVPN procedures and associated
data-plane operation.
The existing standards for MPLS over GRE encapsulation as defined by
[RFC4023] can be used for this purpose; however, when it is used in
conjunction with EVPN the GRE key field SHOULD be present, and SHOULD
be used to provide a 32-bit entropy field. The Checksum and Sequence
Number fields are not needed and their corresponding C and S bits
MUST be set to zero. A PE capable of supporting this encapsulation,
should advertise its EVPN routes along with the Tunnel Encapsulation
extended community indicating MPLS over GRE encapsulation, as
described in previous section.
6 EVPN with Multiple Data Plane Encapsulations
The use of the BGP Encapsulation extended community per [TUNNEL-
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ENCAP] and [RFC5512] allows each NVE in a given EVI to know each of
the encapsulations supported by each of the other NVEs in that EVI.
i.e., each of the NVEs in a given EVI may support multiple data plane
encapsulations. An ingress NVE can send a frame to an egress NVE
only if the set of encapsulations advertised by the egress NVE forms
a non-empty intersection with the set of encapsulations supported by
the ingress NVE, and it is at the discretion of the ingress NVE which
encapsulation to choose from this intersection. (As noted in
section 5.1.3, if the BGP Encapsulation extended community is not
present, then the default MPLS encapsulation or a locally configured
encapsulation is assumed.)
An ingress node that uses shared multicast trees for sending
broadcast or multicast frames MAY maintain distinct trees for each
different encapsulation type.
It is the responsibility of the operator of a given EVI to ensure
that all of the NVEs in that EVI support at least one common
encapsulation. If this condition is violated, it could result in
service disruption or failure. The use of the BGP Encapsulation
extended community provides a method to detect when this condition is
violated but the actions to be taken are at the discretion of the
operator and are outside the scope of this document.
7 Single-Homing NVEs - NVE Residing in Hypervisor
When a NVE and its hosts/VMs are co-located in the same physical
device, e.g., when they reside in a server, the links between them
are virtual and they typically share fate; i.e., the subject
hosts/VMs are typically not multi-homed or if they are multi-homed,
the multi-homing is a purely local matter to the server hosting the
VM and the NVEs, and need not be "visible" to any other NVEs residing
on other servers, and thus does not require any specific protocol
mechanisms. The most common case of this is when the NVE resides on
the hypervisor.
In the sub-sections that follow, we will discuss the impact on EVPN
procedures for the case when the NVE resides on the hypervisor and
the VXLAN (or NVGRE) encapsulation is used.
7.1 Impact on EVPN BGP Routes & Attributes for VXLAN/NVGRE Encapsulation
In scenarios where different groups of data centers are under
different administrative domains, and these data centers are
connected via one or more backbone core providers as described in
[NVO3-FRWK], the RD must be a unique value per EVI or per NVE as
described in [RFC7432]. In other words, whenever there is more than
one administrative domain for global VNI, then a unique RD MUST be
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used, or whenever the VNI value have local significance, then a
unique RD MUST be used. Therefore, it is recommend to use a unique RD
as described in [RFC7432] at all time.
When the NVEs reside on the hypervisor, the EVPN BGP routes and
attributes associated with multi-homing are no longer required. This
reduces the required routes and attributes to the following subset of
four out of eight:
- MAC/IP Advertisement Route
- Inclusive Multicast Ethernet Tag Route
- MAC Mobility Extended Community
- Default Gateway Extended Community
However, as noted in section 8.6 of [RFC7432] in order to enable a
single-homing ingress NVE to take advantage of fast convergence,
aliasing, and backup-path when interacting with multi-homed egress
NVEs attached to a given Ethernet segment, the single-homing ingress
NVE SHOULD be able to receive and process Ethernet AD per ES and
Ethernet AD per EVI routes.
7.2 Impact on EVPN Procedures for VXLAN/NVGRE Encapsulation
When the NVEs reside on the hypervisors, the EVPN procedures
associated with multi-homing are no longer required. This limits the
procedures on the NVE to the following subset of the EVPN procedures:
1. Local learning of MAC addresses received from the VMs per section10.1 of [RFC7432].
2. Advertising locally learned MAC addresses in BGP using the MAC/IP
Advertisement routes.
3. Performing remote learning using BGP per Section 10.2 of
[RFC7432].
4. Discovering other NVEs and constructing the multicast tunnels
using the Inclusive Multicast Ethernet Tag routes.
5. Handling MAC address mobility events per the procedures of Section16 in [RFC7432].
However, as noted in section 8.6 of [RFC7432] in order to enable a
single-homing ingress NVE to take advantage of fast convergence,
aliasing, and back-up path when interacting with multi-homed egress
NVEs attached to a given Ethernet segment, a single-homing ingress
NVE SHOULD implement the ingress node processing of Ethernet AD per
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ES and Ethernet AD per EVI routes as defined in sections 8.2 Fast
Convergence and 8.4 Aliasing and Backup-Path of [RFC7432].
8 Multi-Homing NVEs - NVE Residing in ToR Switch
In this section, we discuss the scenario where the NVEs reside in the
Top of Rack (ToR) switches AND the servers (where VMs are residing)
are multi-homed to these ToR switches. The multi-homing NVE operate
in All-Active or Single-Active redundancy mode. If the servers are
single-homed to the ToR switches, then the scenario becomes similar
to that where the NVE resides on the hypervisor, as discussed in
Section 7, as far as the required EVPN functionality are concerned.
[RFC7432] defines a set of BGP routes, attributes and procedures to
support multi-homing. We first describe these functions and
procedures, then discuss which of these are impacted by the VxLAN
(or NVGRE) encapsulation and what modifications are required. As it
will be seen later in this section, the only EVPN procedure that is
impacted by non-MPLS overlay encapsulation (e.g., VxLAN or NVGRE)
where it provides space for one ID rather than stack of labels, is
that of split-horizon filtering for multi-homed Ethernet Segments
described in section 8.3.1.
8.1 EVPN Multi-Homing Features
In this section, we will recap the multi-homing features of EVPN to
highlight the encapsulation dependencies. The section only describes
the features and functions at a high-level. For more details, the
reader is to refer to [RFC7432].
8.1.1 Multi-homed Ethernet Segment Auto-Discovery
EVPN NVEs (or PEs) connected to the same Ethernet Segment (e.g. the
same server via LAG) can automatically discover each other with
minimal to no configuration through the exchange of BGP routes.
8.1.2 Fast Convergence and Mass Withdraw
EVPN defines a mechanism to efficiently and quickly signal, to remote
NVEs, the need to update their forwarding tables upon the occurrence
of a failure in connectivity to an Ethernet segment (e.g., a link or
a port failure). This is done by having each NVE advertise an
Ethernet A-D Route per Ethernet segment for each locally attached
segment. Upon a failure in connectivity to the attached segment, the
NVE withdraws the corresponding Ethernet A-D route. This triggers all
NVEs that receive the withdrawal to update their next-hop adjacencies
for all MAC addresses associated with the Ethernet segment in
question. If no other NVE had advertised an Ethernet A-D route for
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the same segment, then the NVE that received the withdrawal simply
invalidates the MAC entries for that segment. Otherwise, the NVE
updates the next-hop adjacency list accordingly.
8.1.3 Split-Horizon
If a server is multi-homed to two or more NVEs (represented by an
Ethernet segment ES1) and operating in an all-active redundancy mode,
sends a BUM packet (ie, Broadcast, Unknown unicast, or Multicast) to
one of these NVEs, then it is important to ensure the packet is not
looped back to the server via another NVE connected to this server.
The filtering mechanism on the NVE to prevent such loop and packet
duplication is called "split horizon filtering'.
8.1.4 Aliasing and Backup-Path
In the case where a station is multi-homed to multiple NVEs, it is
possible that only a single NVE learns a set of the MAC addresses
associated with traffic transmitted by the station. This leads to a
situation where remote NVEs receive MAC advertisement routes, for
these addresses, from a single NVE even though multiple NVEs are
connected to the multi-homed station. As a result, the remote NVEs
are not able to effectively load-balance traffic among the NVEs
connected to the multi-homed Ethernet segment. This could be the
case, for e.g. when the NVEs perform data-path learning on the
access, and the load-balancing function on the station hashes traffic
from a given source MAC address to a single NVE. Another scenario
where this occurs is when the NVEs rely on control plane learning on
the access (e.g. using ARP), since ARP traffic will be hashed to a
single link in the LAG.
To alleviate this issue, EVPN introduces the concept of Aliasing.
This refers to the ability of an NVE to signal that it has
reachability to a given locally attached Ethernet segment, even when
it has learnt no MAC addresses from that segment. The Ethernet A-D
route per EVI is used to that end. Remote NVEs which receive MAC
advertisement routes with non-zero ESI SHOULD consider the MAC
address as reachable via all NVEs that advertise reachability to the
relevant Segment using Ethernet A-D routes with the same ESI and with
the Single-Active flag reset.
Backup-Path is a closely related function, albeit it applies to the
case where the redundancy mode is Single-Active. In this case, the
NVE signals that it has reachability to a given locally attached
Ethernet Segment using the Ethernet A-D route as well. Remote NVEs
which receive the MAC advertisement routes, with non-zero ESI, SHOULD
consider the MAC address as reachable via the advertising NVE.
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Furthermore, the remote NVEs SHOULD install a Backup-Path, for said
MAC, to the NVE which had advertised reachability to the relevant
Segment using an Ethernet A-D route with the same ESI and with the
Single-Active flag set.
8.1.5 DF Election
If a host is multi-homed to two or more NVEs on an Ethernet segment
operating in all-active redundancy mode, then for a given EVI only
one of these NVEs, termed the Designated Forwarder (DF) is
responsible for sending it broadcast, multicast, and, if configured
for that EVI, unknown unicast frames.
This is required in order to prevent duplicate delivery of multi-
destination frames to a multi-homed host or VM, in case of all-active
redundancy.
In NVEs where .1Q tagged frames are received from hosts, the DF
election SHOULD BE performed based on host VLAN IDs (VIDs) per
section 8.5 of [RFC7432]. Furthermore, multi-homing PEs of a given
Ethernet Segment MAY perform DF election using configured IDs such as
VNI, EVI, normalized VIDs, and etc. as along the IDs are configured
consistently across the multi-homing PEs.
In GWs where VxLAN encapsulated frames are received, the DF election
is performed on VNIs. Again, it is assumed that for a given Ethernet
Segment, VNIs are unique and consistent (e.g., no duplicate VNIs
exist).
8.2 Impact on EVPN BGP Routes & Attributes
Since multi-homing is supported in this scenario, then the entire set
of BGP routes and attributes defined in [RFC7432] are used. The
setting of the Ethernet Tag field in the MAC Advertisement, Ethernet
AD per EVI, and Inclusive Multicast routes follows that of section5.1.3. Furthermore, the setting of the VNI field in the MAC
Advertisement and Ethernet AD per EVI routes follows that of section5.1.3.
8.3 Impact on EVPN Procedures
Two cases need to be examined here, depending on whether the NVEs are
operating in Single-Active or in All-Active redundancy mode.
First, lets consider the case of Single-Active redundancy mode, where
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the hosts are multi-homed to a set of NVEs, however, only a single
NVE is active at a given point of time for a given VNI. In this case,
the aliasing is not required and the split-horizon filtering may not
be required, but other functions such as multi-homed Ethernet segment
auto-discovery, fast convergence and mass withdraw, backup path, and
DF election are required.
Second, let's consider the case of All-Active redundancy mode. In
this case, out of all the EVPN multi-homing features listed in
section 8.1, the use of the VXLAN or NVGRE encapsulation impacts the
split-horizon and aliasing features, since those two rely on the MPLS
client layer. Given that this MPLS client layer is absent with these
types of encapsulations, alternative procedures and mechanisms are
needed to provide the required functions. Those are discussed in
detail next.
8.3.1 Split Horizon
In EVPN, an MPLS label is used for split-horizon filtering to support
All-Active multi-homing where an ingress NVE adds a label
corresponding to the site of origin (aka ESI Label) when
encapsulating the packet. The egress NVE checks the ESI label when
attempting to forward a multi-destination frame out an interface, and
if the label corresponds to the same site identifier (ESI) associated
with that interface, the packet gets dropped. This prevents the
occurrence of forwarding loops.
Since the VXLAN or NVGRE encapsulation does not include this ESI
label, other means of performing the split-horizon filtering function
MUST be devised. The following approach is recommended for split-
horizon filtering when VXLAN (or NVGRE) encapsulation is used.
Every NVE track the IP address(es) associated with the other NVE(s)
with which it has shared multi-homed Ethernet Segments. When the NVE
receives a multi-destination frame from the overlay network, it
examines the source IP address in the tunnel header (which
corresponds to the ingress NVE) and filters out the frame on all
local interfaces connected to Ethernet Segments that are shared with
the ingress NVE. With this approach, it is required that the ingress
NVE performs replication locally to all directly attached Ethernet
Segments (regardless of the DF Election state) for all flooded
traffic ingress from the access interfaces (i.e. from the hosts).
This approach is referred to as "Local Bias", and has the advantage
that only a single IP address needs to be used per NVE for split-
horizon filtering, as opposed to requiring an IP address per Ethernet
Segment per NVE.
In order to prevent unhealthy interactions between the split horizon
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procedures defined in [RFC7432] and the local bias procedures
described in this document, a mix of MPLS over GRE encapsulations on
the one hand and VXLAN/NVGRE encapsulations on the other on a given
Ethernet Segment is prohibited.
8.3.2 Aliasing and Backup-Path
The Aliasing and the Backup-Path procedures for VXLAN/NVGRE
encapsulation are very similar to the ones for MPLS. In case of MPLS,
Ethernet A-D route per EVI is used for Aliasing when the
corresponding Ethernet Segment operates in All-Active multi-homing,
and the same route is used for Backup-Path when the corresponding
Ethernet Segment operates in Single-Active multi-homing. In case of
VxLAN/NVGRE, the same route is used for the Aliasing and the Backup-
Path with the difference that the Ethernet Tag and VNI fields in
Ethernet A-D per EVI route are set as described in section 5.1.3.
8.3.3 Unknown Unicast Traffic Designation
In EVPN, when an ingress PE uses ingress replication to flood unknown
unicast traffic to egress PEs, the ingress PE uses a different EVPN
MPLS label (from the one used for known unicast traffic) to identify
such BUM traffic. The egress PEs use this label to identify such BUM
traffic and thus apply DF filtering for All-Active multi-homed sites.
In absence of unknown unicast traffic designation and in presence of
enabling unknown unicast flooding, there can be transient duplicate
traffic to All-Active multi-homed sites under the following
condition: the host MAC address is learned by the egress PE(s) and
advertised to the ingress PE; however, the MAC advertisement has not
been received or processed by the ingress PE, resulting in the host
MAC address to be unknown on the ingress PE but be known on the
egress PE(s). Therefore, when a packet destined to that host MAC
address arrives on the ingress PE, it floods it via ingress
replication to all the egress PE(s) and since they are known to the
egress PE(s), multiple copies is sent to the All-Active multi-homed
site. It should be noted that such transient packet duplication only
happens when a) the destination host is multi-homed via All-Active
redundancy mode, b) flooding of unknown unicast is enabled in the
network, c) ingress replication is used, and d) traffic for the
destination host is arrived on the ingress PE before it learns the
host MAC address via BGP EVPN advertisement. In order to prevent such
occurrence of packet duplication (however low probability that may
be), the ingress PE MAY use a flag-bit in the VxLAN header to
indicate BUM traffic type. Bit 6 of flag field in the VxLAN header is
used for this purpose per section 3.1 of [VXLAN-GPE].
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The E-VPN Inclusive Multicast Ethernet Tag (IMET) route is used to
discover the multicast tunnels among the endpoints associated with a
given EVI (e.g., given VNI) for VLAN-based service and a given
<EVI,VLAN> for VLAN-aware bundle service. All fields of this route is
set as described in section 5.1.3. The Originating router's IP
address field is set to the NVE's IP address. This route is tagged
with the PMSI Tunnel attribute, which is used to encode the type of
multicast tunnel to be used as well as the multicast tunnel
identifier. The tunnel encapsulation is encoded by adding the BGP
Encapsulation extended community as per section 5.1.1. For example,
the PMSI Tunnel attribute may indicate the multicast tunnel is of
type PIM-SM; whereas, the BGP Encapsulation extended community may
indicate the encapsulation for that tunnel is of type VxLAN. The
following tunnel types as defined in [RFC6514] can be used in the
PMSI tunnel attribute for VXLAN/NVGRE:
+ 3 - PIM-SSM Tree
+ 4 - PIM-SM Tree
+ 5 - BIDIR-PIM Tree
+ 6 - Ingress Replication
Except for Ingress Replication, this multicast tunnel is used by the
PE originating the route for sending multicast traffic to other PEs,
and is used by PEs that receive this route for receiving the traffic
originated by hosts connected to the PE that originated the route.
In the scenario where the multicast tunnel is a tree, both the
Inclusive as well as the Aggregate Inclusive variants may be used. In
the former case, a multicast tree is dedicated to a VNI. Whereas, in
the latter, a multicast tree is shared among multiple VNIs. For VNI-
based service, the Aggregate Inclusive mode is accomplished by having
the NVEs advertise multiple IMET routes with different Route Targets
(one per VNI) but with the same tunnel identifier encoded in the PMSI
tunnel attribute. For VNI-aware bundle service, the Aggregate
Inclusive mode is accomplished by having the NVEs advertise multiple
IMET routes with different VNI encoded in the Ethernet Tag field, but
with the same tunnel identifier encoded in the PMSI Tunnel attribute.
10 Data Center Interconnections - DCI
For DCI, the following two main scenarios are considered when
connecting data centers running evpn-overlay (as described here) over
MPLS/IP core network:
- Scenario 1: DCI using GWs
- Scenario 2: DCI using ASBRs
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The following two subsections describe the operations for each of
these scenarios.
10.1 DCI using GWs
This is the typical scenario for interconnecting data centers over
WAN. In this scenario, EVPN routes are terminated and processed in
each GW and MAC/IP routes are always re-advertised from DC to WAN but
from WAN to DC, they are not re-advertised if unknown MAC address
(and default IP address) are utilized in NVEs. In this scenario, each
GW maintains a MAC-VRF (and/or IP-VRF) for each EVI. The main
advantage of this approach is that NVEs do not need to maintain MAC
and IP addresses from any remote data centers when default IP route
and unknown MAC routes are used - i.e., they only need to maintain
routes that are local to their own DC. When default IP route and
unknown MAC route are used, any unknown IP and MAC packets from NVEs
are forwarded to the GWs where all the VPN MAC and IP routes are
maintained. This approach reduces the size of MAC-VRF and IP-VRF
significantly at NVEs. Furthermore, it results in a faster
convergence time upon a link or NVE failure in a multi-homed network
or device redundancy scenario, because the failure related BGP routes
(such as mass withdraw message) do not need to get propagated all the
way to the remote NVEs in the remote DCs. This approach is described
in details in section 3.4 of [DCI-EVPN-OVERLAY].
10.2 DCI using ASBRs
This approach can be considered as the opposite of the first approach
and it favors simplification at DCI devices over NVEs such that
larger MAC-VRF (and IP-VRF) tables need to be maintained on NVEs;
whereas, DCI devices don't need to maintain any MAC (and IP)
forwarding tables. Furthermore, DCI devices do not need to terminate
and process routes related to multi-homing but rather to relay these
messages for the establishment of an end-to-end LSP path. In other
words, DCI devices in this approach operate similar to ASBRs for
inter-AS option B - section 10 of [RFC4364]. This requires locally
assigned VNIs to be used just like downstream assigned MPLS VPN label
where for all practical purposes the VNIs function like 24-bit VPN
labels. This approach is equally applicable to data centers (or
Carrier Ethernet networks) with MPLS encapsulation.
In inter-AS option B, when ASBR receives an EVPN route from its DC
over iBGP and re-advertises it to other ASBRs, it re-advertises the
EVPN route by re-writing the BGP next-hops to itself, thus losing the
identity of the PE that originated the advertisement. This re-write
of BGP next-hop impacts the EVPN Mass Withdraw route (Ethernet A-D
per ES) and its procedure adversely. However, it does not impact EVPN
Aliasing mechanism/procedure because when the Aliasing routes (Ether
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A-D per EVI) are advertised, the receiving PE first resolves a MAC
address for a given EVI into its corresponding <ES,EVI> and
subsequently, it resolves the <ES,EVI> into multiple paths (and their
associated next hops) via which the <ES,EVI> is reachable. Since
Aliasing and MAC routes are both advertised per EVI basis and they
use the same RD and RT (per EVI), the receiving PE can associate them
together on a per BGP path basis (e.g., per originating PE) and thus
perform recursive route resolution - e.g., a MAC is reachable via an
<ES,EVI> which in turn, is reachable via a set of BGP paths, thus the
MAC is reachable via the set of BGP paths. Since on a per EVI basis,
the association of MAC routes and the corresponding Aliasing route is
fixed and determined by the same RD and RT, there is no ambiguity
when the BGP next hop for these routes is re-written as these routes
pass through ASBRs - i.e., the receiving PE may receive multiple
Aliasing routes for the same EVI from a single next hop (a single
ASBR), and it can still create multiple paths toward that <ES, EVI>.
However, when the BGP next hop address corresponding to the
originating PE is re-written, the association between the Mass
Withdraw route (Ether A-D per ES) and its corresponding MAC routes
cannot be made based on their RDs and RTs because the RD for Mass
Withdraw route is different than the one for the MAC routes.
Therefore, the functionality needed at the ASBRs and the receiving
PEs depends on whether the Mass Withdraw route is originated and
whether there is a need to handle route resolution ambiguity for this
route. The following two subsections describe the functionality
needed by the ASBRs and the receiving PEs depending on whether the
NVEs reside in a Hypervisors or in TORs.
10.2.1 ASBR Functionality with Single-Homing NVEs
When NVEs reside in hypervisors as described in section 7.1, there is
no multi-homing and thus there is no need for the originating NVE to
send Ethernet A-D per ES or Ethernet A-D per EVI routes. However, as
noted in section 7, in order to enable a single-homing ingress NVE to
take advantage of fast convergence, aliasing, and backup-path when
interacting with multi-homing egress NVEs attached to a given
Ethernet segment, the single-homing NVE SHOULD be able to receive and
process Ethernet AD per ES and Ethernet AD per EVI routes. The
handling of these routes are described in the next section.
10.2.2 ASBR Functionality with Multi-Homing NVEs
When NVEs reside in TORs and operate in multi-homing redundancy mode,
then as described in section 8, there is a need for the originating
multi-homing NVE to send Ethernet A-D per ES route(s) (used for mass
withdraw) and Ethernet A-D per EVI routes (used for aliasing). As
described above, the re-write of BGP next-hop by ASBRs creates
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ambiguities when Ethernet A-D per ES routes are received by the
remote NVE in a different ASBR because the receiving NVE cannot
associated that route with the MAC/IP routes of that Ethernet Segment
advertised by the same originating NVE. This ambiguity inhibits the
function of mass-withdraw per ES by the receiving NVE in a different
AS.
As an example consider a scenario where CE is multi-homed to PE1 and
PE2 where these PEs are connected via ASBR1 and then ASBR2 to the
remote PE3. Furthermore, consider that PE1 receives M1 from CE1 but
not PE2. Therefore, PE1 advertises Eth A-D per ES1, Eth A-D per EVI1,
and M1; whereas, PE2 only advertises Eth A-D per ES1 and Eth A-D per
EVI1. ASBR1 receives all these five advertisements and passes them to
ASBR2 (with itself as the BGP next hop). ASBR2, in turn, passes them
to the remote PE3 with itself as the BGP next hop. PE3 receives these
five routes where all of them have the same BGP next-hop (i.e.,
ASBR2). Furthermore, the two Ether A-D per ES routes received by PE3
have the same info - i.e., same ESI and the same BGP next hop.
Although both of these routes are maintained by the BGP process in
PE3 (because they have different RDs and thus treated as different
BGP routes), information from only one of them is used in the L2
routing table (L2 RIB).
PE1
/ \
CE ASBR1---ASBR2---PE3
\ /
PE2
Figure 1: Inter-AS Option B
Now, when the AC between the PE2 and the CE fails and PE2 sends NLRI
withdrawal for Ether A-D per ES route and this withdrawal gets
propagated and received by the PE3, the BGP process in PE3 removes
the corresponding BGP route; however, it doesn't remove the
associated info (namely ESI and BGP next hop) from the L2 routing
table (L2 RIB) because it still has the other Ether A-D per ES route
(originated from PE1) with the same info. That is why the mass-
withdraw mechanism does not work when doing DCI with inter-AS option
B. However, as described previoulsy, the aliasing function works and
so does "mass-withdraw per EVI" (which is associated with withdrawing
the EVPN route associated with Aliasing - i.e., Ether A-D per EVI
route).
In the above example, the PE3 receives two Aliasing routes with the
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same BGP next hop (ASBR2) but different RDs. One of the Alias route
has the same RD as the advertised MAC route (M1). PE3 follows the
route resolution procedure specified in [RFC7432] upon receiving the
two Aliasing route - ie, it resolves M1 to <ES, EVI1> and
subsequently it resolves <ES,EVI1> to a BGP path list with two paths
along with the corresponding VNIs/MPLS labels (one associated with
PE1 and the other associated with PE2). It should be noted that even
though both paths are advertised by the same BGP next hop (ASRB2),
the receiving PE3 can handle them properly. Therefore, M1 is
reachable via two paths. This creates two end-to-end LSPs, from PE3
to PE1 and from PE3 to PE2, for M1 such that when PE3 wants to
forward traffic destined to M1, it can load balanced between the two
LSPs. Although route resolution for Aliasing routes with the same BGP
next hop is not explicitly mentioned in [RFC7432], this is the
expected operation and thus it is elaborated here.
When the AC between the PE2 and the CE fails and PE2 sends NLRI
withdrawal for Ether A-D per EVI routes and these withdrawals get
propagated and received by the PE3, the PE3 removes the Aliasing
route and updates the path list - ie, it removes the path
corresponding to the PE2. Therefore, all the corresponding MAC routes
for that <ES,EVI> that point to that path list will now have the
updated path list with a single path associated with PE1. This action
can be considered as the mass-withdraw at the per-EVI level. The
mass-withdraw at per-EVI level has longer convergence time than the
mass-withdraw at per-ES level; however, it is much faster than the
convergence time when the withdraw is done on a per-MAC basis.
If a PE becomes detached from a given ES, then in addition to
withdrawing its previously advertised Ethernet AD Per ES routes, it
MUST also withdraw its previously advertised Ethernet AD Per EVI
routes for that ES. For a remote PE that is separated from the
withdrawing PE by one or more EVPN inter-AS option B ASBRs, the
withdrawal of the Ethernet AD Per ES routes is not actionable.
However, a remote PE is able to correlate a previously advertised
Ethernet AD Per EVI route with any MAC/IP Advertisement routes also
advertised by the withdrawing PE for that <ES, EVI, BD>. Hence, when
it receives the withdrawal of an Ethernet AD Per EVI route, it SHOULD
remove the withdrawing PE as a next-hop for all MAC addresses
associated with that <ES, EVI, BD>.
In the previous example, when the AC between PE2 and the CE fails,
PE2 will withdraw its Ethernet AD Per ES and Per EVI routes. When
PE3 receives the withdrawal of an Ethernet AD Per EVI route, it
removes PE2 as a valid next-hop for all MAC addresses associated with
the corresponding <ES, EVI, BD>. Therefore, all the MAC next-hops
for that <ES,EVI, BD> will now have a single next-hop, viz the LSP to
PE1.
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In summary, it can be seen that aliasing (and backup path)
functionality should work as is for inter-AS option B without
requiring any addition functionality in ASBRs or PEs. However, the
mass-withdraw functionality falls back from per-ES mode to per-EVI
mode for inter-AS option B - i.e., PEs receiving mass-withdraw route
from the same AS take action on Ether A-D per ES route; whereas, PEs
receiving mass-withdraw route from different AS take action on Ether
A-D per EVI route.
11 Acknowledgement
The authors would like to thank Aldrin Isaac, David Smith, John
Mullooly, Thomas Nadeau for their valuable comments and feedback. The
authors would also like to thank Jakob Heitz for his contribution on
section 10.2.
12 Security Considerations
This document uses IP-based tunnel technologies to support data
plane transport. Consequently, the security considerations of those
tunnel technologies apply. This document defines support for VXLAN
and NVGRE encapsulations. The security considerations from those
documents as well as [RFC4301] apply to the data plane aspects of
this document.
As with [RFC5512], any modification of the information that is used
to form encapsulation headers, to choose a tunnel type, or to choose
a particular tunnel for a particular payload type may lead to user
data packets getting misrouted, misdelivered, and/or dropped.
More broadly, the security considerations for the transport of IP
reachability information using BGP are discussed in [RFC4271] and
[RFC4272], and are equally applicable for the extensions described
in this document.
If the integrity of the BGP session is not itself protected, then an
imposter could mount a denial-of-service attack by establishing
numerous BGP sessions and forcing an IPsec SA to be created for each
one. However, as such an imposter could wreak havoc on the entire
routing system, this particular sort of attack is probably not of
any special importance.
It should be noted that a BGP session may itself be transported over
an IPsec tunnel. Such IPsec tunnels can provide additional security
to a BGP session. The management of such IPsec tunnels is outside
the scope of this document.
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